Self-aligned contacts

ABSTRACT

A transistor comprises a substrate, a pair of spacers on the substrate, a gate dielectric layer on the substrate and between the pair of spacers, a gate electrode layer on the gate dielectric layer and between the pair of spacers, an insulating cap layer on the gate electrode layer and between the pair of spacers, and a pair of diffusion regions adjacent to the pair of spacers. The insulating cap layer forms an etch stop structure that is self aligned to the gate and prevents the contact etch from exposing the gate electrode, thereby preventing a short between the gate and contact. The insulator-cap layer enables self-aligned contacts, allowing initial patterning of wider contacts that are more robust to patterning limitations.

BACKGROUND

Metal-oxide-semiconductor (MOS) transistors, such as MOS field effecttransistors (MOSFET), are used in the manufacture of integratedcircuits. MOS transistors include several components, such as a gateelectrode, gate dielectric layer, spacers, and diffusion regions such assource and drain regions. An interlayer dielectric (ILD) is typicallyformed over the MOS transistor and covers the diffusion regions.

Electrical connections are made to the MOS transistor by way of contactplugs that are typically formed of a metal such as tungsten. The contactplugs are fabricated by first patterning the ILD layer to form vias downto the diffusion regions. The patterning process is generally aphotolithography process. Next, metal is deposited in the vias to formthe contact plugs. A separate contact plug is formed down to the gateelectrode using the same or a similar process.

One problem that can occur during the fabrication of a contact plug isthe formation of a contact-to-gate short. A contact-to-gate short is ashort circuit that occurs when the contact plug is misaligned and comesinto electrical contact with the gate electrode. One conventionalapproach to preventing contact-to-gate shorts is by controllingregistration and critical dimensions (CDs). Unfortunately, fortransistors with gate pitches (gate length+space) at or below 100nanometers (nm), CD control for gate and contact dimensions needs to beless than 10 nm and the registration control between gate and contactlayers also needs to be less than 10 nm to deliver a manufacturableprocess window. Thus, the likelihood of a contact shorting to a gate isvery high. This problem becomes more prevalent as transistor gate pitchdimensions are scaled down further because the critical dimensionsbecome much smaller.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a substrate and two conventional MOS transistorswith a correctly aligned trench contact.

FIG. 1B illustrates a misaligned trench contact formed to a diffusionregion of the MOS transistors, resulting in a contact-to-gate short.

FIG. 2A illustrates a substrate and two MOS transistors havinginsulator-cap layers atop their respective metal gate electrodes inaccordance with one implementation of the invention.

FIG. 2B illustrates a correctly aligned trench contact formed betweentwo MOS transistors of the invention having insulator-cap layers.

FIG. 2C illustrates a misaligned trench contact formed between two MOStransistors of the invention having insulator-cap layers, where themisalignment does not result in a contact-to-gate short.

FIGS. 3A to 3C illustrate an insulator-cap layer formed after areplacement metal gate process, in accordance with an implementation ofthe invention.

FIGS. 4A to 4C illustrate an insulator-cap layer formed after areplacement metal gate process, in accordance with anotherimplementation of the invention.

FIGS. 5A to 5I illustrate a fabrication process for an insulator-caplayer that extends over the spacers of a MOS transistor, in accordancewith an implementation of the invention.

FIGS. 6A to 6F illustrate a fabrication process for a metal gateelectrode having a stepped profile, in accordance with an implementationof the invention.

FIGS. 7A to 7C illustrate MOS transistors having both metal gateelectrodes with stepped profiles and insulator-cap layers that extendover the spacers, in accordance with an implementation of the invention.

FIG. 8A to 8F illustrate contact sidewall spacers in accordance with animplementation of the invention.

FIGS. 9A to 9D illustrate a fabrication process to form aninsulating-cap atop a metal gate electrode in accordance with animplementation of the invention.

FIGS. 10A to 10G illustrate a fabrication process to form a metal studand insulating spacers atop a trench contact in accordance with animplementation of the invention.

DETAILED DESCRIPTION

Described herein are systems and methods of reducing the likelihood ofcontact-to-gate shorts during the fabrication ofmetal-oxide-semiconductor (MOS) transistors. In the followingdescription, various aspects of the illustrative implementations will bedescribed using terms commonly employed by those skilled in the art toconvey the substance of their work to others skilled in the art.However, it will be apparent to those skilled in the art that thepresent invention may be practiced with only some of the describedaspects. For purposes of explanation, specific numbers, materials andconfigurations are set forth in order to provide a thoroughunderstanding of the illustrative implementations. However, it will beapparent to one skilled in the art that the present invention may bepracticed without the specific details. In other instances, well-knownfeatures are omitted or simplified in order not to obscure theillustrative implementations.

Various operations will be described as multiple discrete operations, inturn, in a manner that is most helpful in understanding the presentinvention, however, the order of description should not be construed toimply that these operations are necessarily order dependent. Inparticular, these operations need not be performed in the order ofpresentation.

FIG. 1A illustrates a substrate 100 and two MOS transistors 101. The MOStransistors 101 include gate electrodes 102, gate dielectric layers 104,and spacers 108. Diffusion regions 106 are formed in the substrate 100.Interlayer dielectrics (ILD), such as ILD layers 110 a and 110 b, aredeposited in the regions between and around the two MOS transistors 101.

FIG. 1A also illustrates a trench contact 200 that is formed through theILD layers 110 a/b down to the diffusion region 106. The trench contact200 is typically formed using a photolithography patterning processfollowed by a metal deposition process. Photolithography patterningprocesses and metal deposition processes are well known in the art. Thephotolithography patterning process etches a trench opening through theILD layers 110 a/b down to the diffusion region 106. The metaldeposition process, such as electroplating, electroless plating,chemical vapor deposition, physical vapor deposition, sputtering, oratomic layer deposition, fills the trench opening with a metal such astungsten or copper. A metal liner is often deposited prior to the metal,such as a tantalum or tantalum nitride liner. A planarization process,such as chemical-mechanical polishing (CMP), is used to remove anyexcess metal and complete the fabrication of the trench contact 200.

It should be noted that in alternate implementations of the invention,via contacts may be used instead of trench contacts. Thus, the contactopening may be either a trench shape or a via shape, depending on thepatterning process used or the needs of a particular integrated circuitprocess. The implementations of the invention described herein willrefer to contact trench openings and trench contacts, but it should benoted that via openings and via contacts (also known as contact plugs orvia plugs) can be used instead of contact trench openings and trenchcontacts in any of these implementations.

As integrated circuit technology advances, transistor gate pitchesprogressively scale down. This gate pitch scaling has resulted in anumber of new, problematic issues, one of which is increased parasiticcapacitance (denoted by the “C” in FIG. 1A) caused by relatively tightspacing between the trench contact 200 and the diffusion region 106 onone side and the gate electrode 102 on the other. The spacers 108 tendto provide the bulk of the separation between the trench contact200/diffusion region 106 and the gate electrodes 102. Conventionalspacer materials, such as silicon nitride, do little to reduce thisparasitic capacitance. Unfortunately, parasitic capacitance degradestransistor performance and increases chip power.

Another problematic issue caused by gate pitch scaling is the formationof contact-to-gate (CTG) shorts. The fabrication process for the trenchcontact 200 is designed to prevent the trench contact 200 from cominginto physical contact with the metal gate electrode 102. When suchcontact occurs, a CTG short is created that effectively ruins the MOStransistor. CTG shorts have become a major yield limiter as transistorgate pitches have scaled down below 100 nanometers (nm).

Current methods to reduce CTG shorts include controlling registrationand patterning contacts with smaller critical dimensions. However, asgate pitch has scaled down, the registration requirements are becomingvery difficult to meet with existing technology. For instance,transistors with gate pitches at or below 100 nm require CD control andlayer registration control of less than 10 nm to deliver amanufacturable process window. Thus, the likelihood of a contactshorting to a gate is very high.

FIG. 1B illustrates what happens when the trench contact 200 ismisaligned. The same photolithography processes are used, but as shown,the trench contact 200 is formed at a location that is not completelywithin the area between the two spacers 108. The misalignment causes thetrench contact 200 to be in physical contact with one of the gateelectrodes 102, thereby creating a contact-to-gate short.

In accordance with implementations of the invention, an insulator-cappedgate electrode may be used to minimize the likelihood of contact-to-gateshorts. In one implementation, the insulator-cap layer is formed atopthe gate electrode 102 and within the spacers 108 of the MOS transistor101. In some implementations of the invention, the insulator-cap canconsume a significant portion of the volume that exists between thespacers. For instance, the insulator-cap can consume anywhere from 10%to 80% of the volume that exists between the spacers, but will generallyconsume between 20% and 50% of that volume. The gate electrode and gatedielectric layer consume the majority of the remaining volume. Materialsthat may be used to form the insulator-cap are described below.

FIG. 2A illustrates an insulator-capped metal gate electrode inaccordance with one implementation of the invention. A substrate 100 isshown in FIG. 2A upon which MOS transistors 101 are formed. Thesubstrate 100 may be a crystalline semiconductor substrate formed usinga bulk silicon substrate or a silicon-on-insulator substructure. Inother implementations, the semiconductor substrate may be formed usingalternate materials, which may or may not be combined with silicon, thatinclude but are not limited to germanium, indium antimonide, leadtelluride, indium arsenide, indium phosphide, gallium arsenide, galliumantimonide, or other Group III-V materials. Although a few examples ofmaterials from which the substrate may be formed are described here, anymaterial that may serve as a foundation upon which a semiconductordevice may be built falls within the spirit and scope of the presentinvention.

Each MOS transistor 101 can be a planar transistor, as shown in FIG. 2A,or can be a nonplanar transistor, such as a double-gate or trigatetransistor. Although the implementations described herein illustrateplanar transistors, the invention is not limited to planar transistors.Implementations of the invention may also be used on nonplanartransistors, including but not limited to FinFET or trigate transistors.Each MOS transistor 101 includes a gate stack formed of three layers: agate dielectric layer 104, a gate electrode layer 102, and aninsulator-cap layer 300. The gate dielectric layer 104 may be formed ofa material such as silicon dioxide or a high-k material. Examples ofhigh-k materials that may be used in the gate dielectric layer 104include, but are not limited to, hafnium oxide, hafnium silicon oxide,lanthanum oxide, lanthanum aluminum oxide, zirconium oxide, zirconiumsilicon oxide, tantalum oxide, titanium oxide, barium strontium titaniumoxide, barium titanium oxide, strontium titanium oxide, yttrium oxide,aluminum oxide, lead scandium tantalum oxide, and lead zinc niobate. Insome embodiments, the gate dielectric layer 104 may have a thicknessbetween around 1 Angstrom (Å) and around 50 Å. In further embodiments,additional processing may be performed on the gate dielectric layer 104,such as an annealing process to improve its quality when a high-kmaterial is used.

The gate electrode layer 102 is formed on the gate dielectric layer 104and may consist of at least a P-type workfunction metal or an N-typeworkfunction metal, depending on whether the transistor is to be a PMOSor an NMOS transistor. In some implementations, the gate electrode layer102 may consist of two or more metal layers, where at least one metallayer is a workfunction metal layer and at least one metal layer is afill metal layer.

For a PMOS transistor, metals that may be used for the gate electrodeinclude, but are not limited to, ruthenium, palladium, platinum, cobalt,nickel, and conductive metal oxides, e.g., ruthenium oxide. A P-typemetal layer will enable the formation of a PMOS gate electrode with aworkfunction that is between about 4.9 eV and about 5.2 eV. For an NMOStransistor, metals that may be used for the gate electrode include, butare not limited to, hafnium, zirconium, titanium, tantalum, aluminum,alloys of these metals, and carbides of these metals such as hafniumcarbide, zirconium carbide, titanium carbide, tantalum carbide, andaluminum carbide. An N-type metal layer will enable the formation of anNMOS gate electrode with a workfunction that is between about 3.9 eV andabout 4.2 eV.

The insulator-cap layer 300 is formed on the gate electrode layer 102and may be formed of materials that include, but are not limited to,silicon nitride, silicon oxide, silicon carbide, silicon nitride dopedwith carbon, silicon oxynitride, other nitride materials, other carbidematerials, aluminum oxide, other oxide materials, other metal oxides,boron nitride, boron carbide, and other low-k dielectric materials orlow-k dielectric materials doped with one or more of carbon, nitrogen,and hydrogen. The insulator-cap layer 300 is described in more detailbelow.

A pair of spacers 108 brackets the gate stack. The spacers 108 may beformed from a material such as silicon nitride, silicon oxide, siliconcarbide, silicon nitride doped with carbon, and silicon oxynitride.Processes for forming spacers are well known in the art and generallyinclude deposition and etching process steps.

Diffusion regions 106 are formed within the substrate 100 adjacent tothe gate stacks of the MOS transistors 101. For each MOS transistor 101,one adjacent diffusion region 106 functions as a source region and theother adjacent diffusion region 106 functions as a drain region.

The diffusion region 106 may be formed using methods or processes thatare well known in the art. In one implementation, dopants such as boron,aluminum, antimony, phosphorous, or arsenic may be implanted into thesubstrate 100 to form the diffusion regions 106. In anotherimplementation, the substrate 100 may first be etched to form recessesat the locations of the diffusion regions 106. An epitaxial depositionprocess may then be carried out to fill the recesses with a siliconalloy such as silicon germanium or silicon carbide, thereby forming thediffusion regions 106. In some implementations the epitaxially depositedsilicon alloy may be doped in situ with dopants such as boron, arsenic,or phosphorous. In further implementations, alternate materials may bedeposited into the recesses to form the diffusion regions 106.

One or more ILD layers 110 a/b are deposited over the MOS transistors101. The ILD layers 110 a/b may be formed using dielectric materialsknown for their applicability in integrated circuit structures, such aslow-k dielectric materials. Examples of dielectric materials that may beused include, but are not limited to, silicon dioxide (SiO₂), carbondoped oxide (CDO), silicon nitride, organic polymers such asperfluorocyclobutane or polytetrafluoroethylene, fluorosilicate glass(FSG), and organosilicates such as silsesquioxane, siloxane, ororganosilicate glass. The ILD layers 110 a/b may include pores or othervoids to further reduce their dielectric constant.

Fabrication of a trench contact 200, also referred to as contactpatterning, involves at least a photolithography process and an etchingprocess. The photolithography process forms a photoresist hard mask thatdefines the location of the trench contact 200. The process begins bydepositing a photoresist material on the ILD layer 110 b. The depositedphotoresist layer is exposed to ultraviolet radiation through apatterned optical mask, wherein the pattern defines the trench contact200. The photoresist layer is then developed to create a photoresisthard mask layer that includes an opening where the trench contact 200 isto be formed. It should be noted that photolithography processes arewell known in the art and this description is simply a brief overview ofa typical photolithography process. Many intermediate steps, such asbaking and alignment steps, have been omitted.

Once the photoresist hard mask is in place defining the trench contact200, an etching process is carried out. The etchant etches portions ofthe ILD layer 110 a/b that are left exposed by openings in thephotoresist hard mask, such as the opening for the trench contact 200.The etchant therefore etches a trench opening down to the diffusionregion 106. The etching process used may be a conventional chemical wetetch process or a plasma dry etch process. The etching process iscarried out for a duration of time, denoted as T_(ETCH), that issufficient to etch the ILD layer 110 all the way down to the diffusionregion 106. The etched trench opening is then filled with one or moremetals, as described above, to form the trench contact 200.

In accordance with implementations of the invention, the insulator-caplayer 300 has a thickness that is sufficient to protect the metal gateelectrode 102 from being exposed during fabrication of the trenchcontact 200 should the contact trench opening be aligned over theinsulator-cap layer. Furthermore, the insulator-cap layer 300 has athickness that is sufficient to electrically isolate the metal gateelectrode 102 from the trench contact 200 after the trench contact 200is formed. In one implementation of the invention, this thickness canrange from 5 nm to 50 nm. In another implementation, the height of theinsulator-cap layer can account for 20% to 80% of the overall height ofthe gate stack. The etching process used to form the contact trenchopening is selective to the insulator-cap layer 300. This means the wetor dry etch chemistry will etch the material of the ILD layer 110 a/bbut will selectively stop and self align to the insulator-cap layer 300and the sidewall spacers 108.

In accordance with implementations of the invention, the insulator-caplayer 300 also has a thickness that is sufficient to withstand theetching process for the entirety of T_(ETCH) without exposing theunderlying metal gate electrode 102. Stated differently, theinsulator-cap layer 300 has an initial thickness sufficient to withstandthe etching process for a duration of time needed to etch the ILD layer110 a/b all the way down to the diffusion region 106 without any portionof the insulator-cap layer 300 being reduced to a thickness that wouldpermit electrical conductivity between the metal gate electrode 102 andthe subsequently formed trench contact 200. After the etching process,the combination of the insulator-cap layer 300 and the spacers 108electrically isolates the metal gate electrode 102 from the trenchcontact 200, thereby eliminating CTG shorts.

There are several different ways to form the insulator-cap layer 300 ofthe invention. In one implementation of the invention, where the gateelectrode 102 is formed using a gate-first process, a blanket dielectriclayer is initially deposited on a substrate. Next, a blanket electrodelayer is deposited atop the dielectric layer. Finally, a blanketinsulator layer is formed atop the electrode layer. The depositionprocesses that are used to deposit the dielectric layer, the electrodelayer, and the insulator layer are well known in the art and mayinclude, but are not limited to, processes such as electroplating,electroless plating, chemical vapor deposition, atomic layer deposition,physical vapor deposition, and sputtering. The three layers are thenetched using conventional patterning processes, such as photolithographyprocesses, to form a gate stack consisting of a gate dielectric layer104, a gate electrode layer 102, and an insulator-cap layer 300. Spacers108 and diffusion regions 106 are then formed on opposing sides of thegate stack. An ILD layer 110 a is deposited over the gate stack, thespacers 108, and the diffusion region 110. A trench contact 200 may thenbe formed as described above.

In an alternate implementation of a gate-first process, a blanketdielectric layer and a blanket electrode layer may be deposited andpatterned to form a gate stack that consists of the gate dielectriclayer 104 and the gate electrode 102. A pair of spacers 108 anddiffusion regions 106 may be formed on either side of the gate stack.Next, an etching process may be carried out to recess the metal gateelectrode 102 within the spacers 108, thereby reducing the thickness ofthe metal gate electrode 102. The recessing of the metal gate electrode102 results in the formation of a trench between the spacers 108 wherethe bottom surface of the trench corresponds to the top surface of therecessed metal gate electrode 102. The metal etch process is followed byan insulator material deposition process that deposits a blanket layerof insulator material and fills the trench between the spacers 108. Apolishing process, such as a chemical mechanical planarization process,is used to polish down the insulator material layer and substantiallyremove any insulator material that is outside of the spacers 108. Theremoval of this excess insulator material yields an insulator-cap layer300 that is substantially contained within the spacers 108.

In another implementation of the invention, a gate-last process, such asa replacement metal gate process, is used to form the gate electrode. Inthis implementation, a blanket dielectric layer and a blanket dummyelectrode layer may be initially deposited and patterned to form a gatestack that consists of the gate dielectric layer 104 and a dummy gateelectrode (not shown). It should be noted that the term “dummy” is usedto indicate that this layer is sacrificial in nature. The materials usedin dummy layers may or may not be the same materials that are used innon-dummy layers. For instance, the dummy electrode layer may consist ofpolysilicon, which is used in real gate electrodes. A pair of spacers108 and diffusion regions 106 may be formed on either side of the gatestack. Next, the dummy gate electrode may be etched out to form a trenchbetween the spacers 108 and atop the gate dielectric layer 104. Anelectrode metal layer may then be deposited to fill the trench. Theelectrode metal layer may be polished down to remove metal outside ofthe spacers 108 and to confine the electrode metal to the trench betweenthe spacers 108, thereby forming a metal gate electrode 102.

As described above, an etching process is carried out to recess themetal gate electrode 102 within the spacers 108. The recessing of themetal gate electrode 102 results in the formation of a trench betweenthe spacers 108. An insulator material deposition process fills thetrench and a polishing process is used to polish down the insulatormaterial layer and substantially remove any insulator material that isoutside of the spacers 108. This yields an insulator-cap layer 300 thatis substantially contained within the spacers 108.

FIG. 2B illustrates a trench contact 200 that is correctly alignedbetween two MOS transistors having insulator-cap layers 300. In thisinstance the insulator-cap 300 is not used.

FIG. 2C illustrates a misaligned trench contact 200 formed between twoMOS transistors having insulator-cap layers 300. As shown, a portion ofthe misaligned trench contact 200 is situated directly over the gateelectrode 102. Unlike the prior art transistors shown in FIG. 1B,however, a CTG short is avoided due to the use of the insulator-caplayer 300. The insulator-cap layer 300 electrically isolates the metalgate electrode 102 from the misaligned trench contact 200, allowing thetrench contact 200 to be “self-aligned”.

FIGS. 3A to 3C illustrate a slight variation on the transistors of FIG.2A. In FIG. 3A, a different implementation of a replacement metal gateprocess is used to form the transistors. In this implementation, ablanket dummy dielectric layer and a blanket dummy electrode layer aredeposited on a substrate. Here, the dummy electrode layer may consist ofpolysilicon and the dummy dielectric layer may consist of silicondioxide, both of which are used in real gate electrodes and real gatedielectric layers. These two dummy layers are etched to form a gatestack that consists of a dummy gate dielectric layer and a dummy gateelectrode layer. Spacers 108 and diffusion regions 106 are then formedon opposing sides of the gate stack. An ILD layer 110 a is depositedover the gate stack, spacers 108, and diffusion regions 106. The ILDlayer 110 a is planarized to expose the dummy electrode layer.

Next, the dummy electrode layer and the dummy gate dielectric layer areremoved using one or more etching processes. The removal of the dummylayers produces a trench between the spacers 108. The substrate 100forms a bottom surface of the trench. A new high-k gate dielectric layer104 is deposited into the trench using a chemical vapor depositionprocess or an atomic layer deposition process. The high-k gatedielectric layer 104 is deposited along the bottom and sidewalls of thetrench, thereby forming a “U” shaped gate dielectric layer 104, as shownin FIG. 3A. Next, a metal gate electrode layer 102 is deposited atop thehigh-k gate dielectric layer 104. Processes for forming the metal gateelectrode 102 are well known in the art.

In accordance with implementations of the invention, the final metalgate electrode 102 does not fill the trench in its entirety. In oneimplementation, the metal gate electrode 102 may initially fill thetrench in its entirety, but a subsequent etching process may be used torecess the metal gate electrode 102. In another implementation, themetal gate electrode deposition process only partially fills the trenchwith the metal gate electrode 102. In both implementations, a trenchremains above the final metal gate electrode 102 between the spacers108.

Finally, an insulator material deposition process is used to deposit ablanket layer of insulator material that fills the trench between thespacers 108. A polishing process, such as a chemical mechanicalplanarization process, is then used to polish down the insulatormaterial layer and remove substantially any insulator material that isoutside of the spacers 108. The removal of this excess insulator yieldsan insulator-cap layer 300 that is substantially confined within thespacers 108. As shown in FIG. 3A, the insulator-cap 300 is also confinedwithin the sidewall portions of the gate dielectric layer 104.

FIG. 3B illustrates a trench contact 200 that is correctly alignedbetween two MOS transistors having insulator-cap layers 300. FIG. 3Cillustrates a misaligned trench contact 200 formed between two MOStransistors having insulator-cap layers 300. Again, a portion of themisaligned trench contact 200 is situated directly over the gateelectrode 102. A CTG short is avoided due to the use of theinsulator-cap layer 300, which electrically isolates the metal gateelectrode 102 from the misaligned trench contact 200.

FIGS. 4A to 4C illustrate a slight variation on the transistors of FIG.3A. In FIG. 4A, a replacement gate process is used again to formtransistors having a “U” shaped gate dielectric layer 104. The gateelectrode layer 102 and the gate dielectric layer 104 are initiallyformed using the same processes detailed above for FIG. 3A. Unlike FIG.3A, in this implementation, both the “U” shaped gate dielectric layer104 and the metal gate electrode 102 are recessed prior to fabricationof the insulator-cap layer 300. One or more etching processes may beused to recess both structures. The insulator-cap 300 is then formedusing the same process described above for FIG. 3A and is situated atopboth the gate electrode 102 and portions of the gate dielectric layer104, as shown in FIG. 4A. FIG. 4B illustrates a trench contact 200 thatis correctly aligned between two MOS transistors having insulator-caplayers 300. FIG. 4C illustrates a misaligned trench contact 200 formedbetween two MOS transistors having insulator-cap layers 300. Again, aportion of the misaligned trench contact 200 is situated directly overthe gate electrode 102. A CTG short is avoided due to the use of theinsulator-cap layer 300, which electrically isolates the metal gateelectrode 102 from the misaligned trench contact 200.

FIGS. 5A to 5F illustrate the fabrication of an alternate insulator-caplayer that may be used with a MOS transistor. Initially, FIG. 5Aillustrates two MOS transistors that include a dummy gate electrode 500and a dummy gate dielectric layer 502. Also shown are a pair of spacers108 that are generally formed of silicon nitride.

In accordance with implementations of the invention, one or multipleetching processes are carried out to partially recess both the dummygate electrode layer 500 and the spacers 108. This dual recess is shownin FIG. 5B. The etch chemistry used to recess the dummy gate electrode500 may differ from the etch chemistry used to recess the spacers 108.The etching processes used may be wet etches, dry etches, or acombination. When the dummy gate electrode 500 and the spacers 108 havebeen recessed, a trench 503 a is formed within the ILD layer 110 a wherethe top surfaces of the dummy gate electrode 500 and the spacers 108form the bottom of the trench.

Moving to FIG. 5C, one or more etching processes are carried out tocompletely remove the dummy gate electrode 500 as well as the dummy gatedielectric 502. Etching processes to completely remove the dummy gateelectrode 500 and dummy gate dielectric are well known in the art.Again, these etches may be wet, dry, or a combination. As shown in FIG.5C, the trench 503 a is now much deeper and has a cross-section profilethat is relatively wide at the top of the trench 503 a and relativelynarrow at the bottom of the trench 503 a. The dummy gate electrode 500and dummy gate dielectric 502 are removed in their entirety, therebyexposing the top of the substrate 100.

In FIG. 5D, a gate dielectric layer 104 and a metal gate electrode layer102 are deposited in the trench 503 a. A conformal deposition process,such as a CVD or an ALD process, is generally used for the deposition ofthe gate dielectric layer 104, resulting in a conformal dielectric layer104 that covers the sidewalls and bottom surface of the trench 503 a.The metal gate electrode layer 102 fills the remainder of the trench 503a. In some implementations of the invention, the metal gate electrodelayer 102 may consist of two or more layers of metal, for instance, awork function metal layer and a fill metal layer.

In a replacement metal gate process flow, it is very challenging to fillnarrow gate trenches with metal gate materials, particularly withtransistors having gate widths at or below 22 nm. The process flowdescribed here in FIGS. 5A to 5D enhances the intrinsic fillcharacteristics by widening the trench openings at the top withoutaffecting the narrow trench widths at the bottom. Thus, thecross-section profile of the trench 503 a, with its relatively wideopening at the top, results in an improved metal gate electrodedeposition with fewer voids or other defects.

Next, the metal gate electrode layer 102 and the gate dielectric layer104 are recessed as shown in FIG. 5E, forming a trench 503 b. Again, oneor more etching processes, either wet or dry, may be used to recess boththe gate electrode layer 102 and the gate dielectric layer 104. The etchprocesses used must be selective to the ILD layer 110 a. The metal gateelectrode 102 is recessed until its top surface is even with or belowthe top surfaces of the spacers 108. Although portions of the metal gateelectrode 102 are on top of the spacers 108 in FIG. 5D, it is importantthat no portion of the metal gate electrode 102 remain above the top ofthe spacers 108 after the recessing of the metal gate 102 in FIG. 5E.This is because any portion of the metal gate electrode 102 that remainsatop the spacers 108 may end up forming a CTG short with a misalignedtrench contact.

Moving to FIG. 5F, an insulator material deposition process fills thetrench 503 b and a polishing process is used to polish down theinsulator material layer and substantially remove any insulator materialthat is outside of the trench 503 b. This yields an insulator-cap layer504 that is substantially contained within the trench 503 b. Theinsulator-cap layer 504 has the appearance of a mushroom top as itextends laterally above the spacers 108. The insulator-cap layer 504improves contact-to-gate margin by extending over the gate spacer 108.The insulator-cap layer 504 may be formed of materials that include, butare not limited to, silicon nitride, silicon oxide, silicon carbide,silicon nitride doped with carbon, silicon oxynitride, other nitridematerials, other carbide materials, aluminum oxide, other oxidematerials, other metal oxides, and low-k dielectric materials.

FIG. 5G illustrates the deposition of an additional ILD layer 110 b thatcovers the insulator-cap layers 504 and sits atop the first ILD layer110 a. FIG. 5H illustrates a trench contact 200 that has been fabricateddown to the diffusion region 106 through the ILD layers 110 a and 110 b.The trench contact 200 of FIG. 5H has been correctly aligned between thespacers 108 of adjacent transistors.

FIG. 5I illustrates a trench contact 200 that is misaligned. As shown,even though the trench contact 200 is situated on top of the metal gateelectrode 102, the insulating-cap layer 504 protects the metal gateelectrode 102 and prevents a CTG short from forming by electricallyisolating the metal gate electrode 102 from the misaligned trenchcontact 200.

Another advantage provided by the insulating-cap layer 504 addresses theparasitic capacitance issue discussed above in relation to FIG. 1A.Parasitic capacitance issues are caused by the relatively tight spacingbetween the trench contact 200 and the diffusion region 106 on one sideand the gate electrode 102 on the other side. The spacers 108 tend toprovide the bulk of the separation between the trench contact200/diffusion region 106 and the gate electrodes 102, but conventionalspacer materials, such as silicon nitride, do little to reduce thisparasitic capacitance. Nevertheless, silicon nitride is still usedbecause the etching process that creates a contact trench opening forthe trench contact 200 is selective to silicon nitride.

In accordance with this implementation of the invention, materials otherthan silicon nitride may be used in the spacers 108. Here, the laterallyextending insulating-cap layer 504 protects the underlying spacers 108during etching processes used to fabricate the trench contact 200. Theseetching processes are generally anisotropic processes, therefore, theetch chemistry need only be selective to the insulating-cap layer 504.The insulating-cap layer 504 can then shield the underlying spacers 108.So with an anisotropic process, the use of the insulating-cap layer 504means the etch chemistry does not necessarily need to be selective tothe material used in the spacers 108. This removes any constraints onthe choice of spacer material and enables the use of materials that areoptimized for capacitance. For instance, materials such as siliconoxynitride (SiON), carbon-doped silicon oxynitride (SiOCN), or low-kdielectric materials may be used in the spacers 108 to reduce issueswith parasitic capacitance.

FIGS. 6A to 6F illustrate the formation of a stepped metal gateelectrode in conjunction with an insulating-cap layer in accordance withan implementation of the invention. Initially, FIG. 6A illustrates twoMOS transistors that include a dummy gate electrode 500 and a dummy gatedielectric layer 502. Moving to FIG. 6B, one or more etching processesare carried out to completely remove the dummy gate electrode 500 aswell as the dummy gate dielectric 502. Etching processes to completelyremove the dummy gate electrode 500 and dummy gate dielectric are wellknown in the art. The dummy gate electrode 500 and dummy gate dielectric502 are removed in their entirety, thereby exposing the top of thesubstrate 100.

FIG. 6C illustrates the deposition of dual metal gate electrode layers,a conformal metal gate electrode layer 102 a and a second metal layer102 b that may or may not be conformal. The initial metal gate electrodelayer 102 a may be deposited using a conformal deposition process suchas chemical vapor deposition or atomic layer deposition. Otherprocesses, such as physical vapor deposition or sputtering, may also beused. The second metal gate electrode 102 b is deposited using aconventional deposition process such as chemical vapor deposition,atomic layer deposition, physical vapor deposition, sputtering, or evenprocesses such as electroplating or electroless plating since aconformal layer is not needed for layer 102 b.

The initial metal gate electrode layer 102 a is typically a workfunctionmetal layer and can be formed using any of the workfunction metalsdescribed above. The second metal gate electrode layer 102 b may be asecond workfunction metal layer or it may be a low resistance fill metallayer such as aluminum, tungsten, or copper. In accordance withimplementations of the invention, the metal used in the metal gateelectrode 102 a has different etch properties than the metal used in themetal gate electrode 102 b.

Moving to FIG. 6D, the dual metal gate electrode layers 102 a and 102 bare etched and recessed to form trenches 600 in which insulating caplayers may be fabricated. In accordance with an implementation of theinvention, the etching process removes a larger portion of metal layer102 a than metal layer 102 b. This yields a stepped or bulleted profilefor the metal gate electrode 102, as shown in FIG. 6D. A middle portionof the overall metal gate electrode 102 is relatively thicker than theouter edge portions of the overall metal gate electrode 102. Stateddifferently, a middle portion of the metal gate electrode 102 has arelatively larger height than side portions of the metal gate electrode102. This stepped profile for the metal gate electrode 102 providesadvantages as explained below in FIG. 6F.

In one implementation, a single etching process is used that etches themetal gate electrode layer 102 a at a faster rate than the metal gateelectrode layer 102 b. In other words, the etch chemistry is moreselective to the metal gate electrode 102 b. In another implementation,two etching processes may be used, one for metal layer 102 a and anotherfor metal layer 102 b. If two etching processes are used, a largerportion of metal layer 102 a must be removed relative to metal layer 102b. Thus in one implementation, the first of the two etching processesmay be selective to the metal layer 102 b and the second of the twoetching processes may be selective to the metal layer 102 a. The etchingprocesses used may be wet etch, dry etch, or a combination of both. Itwill be appreciated by those of ordinary skill in the art that foralmost any arbitrary pair of metals used in metal layers 102 a and 102b, it is possible to find a wet or dry chemical etch that willdifferentiate between the two metals.

As shown in FIG. 6E, an insulator material deposition process fills thetrenches 600 and a polishing process is used to polish down theinsulator material layer and substantially remove any insulator materialthat is outside of the trench 600. This yields an insulator-cap layer602 that is substantially contained within the trench 600. Theinsulator-cap layer 602 is relatively thick at its outer edges andrelatively thin at its middle portion due to the stepped profile of themetal gate electrode 102. The insulator-cap layer 602 may be formed ofmaterials that include, but are not limited to, silicon nitride, siliconoxide, silicon carbide, silicon nitride doped with carbon, siliconoxynitride, other nitride materials, other carbide materials, aluminumoxide, other oxide materials, other metal oxides, and low-k dielectricmaterials.

FIG. 6F illustrates a trench contact 200 that is misaligned. As shown,even though the trench contact 200 is situated on top of the metal gateelectrode 102, the insulating-cap layer 602 protects the metal gateelectrode 102 and prevents a CTG short from forming by electricallyisolating the metal gate electrode 102 from the misaligned trenchcontact 200. The stepped profile of the metal gate electrode 102provides at least two advantages. First, the stepped profile causes thethick portion of the insulator-cap layer 602 to be positioned betweenthe metal gate electrode 102 and the trench contact 200, therebyproviding strong electrical isolation. Second, the stepped profileallows the middle portion of the metal gate electrode 102 to remainthick, thereby lowering the electrical resistance of the metal gateelectrode 102 by increasing its metal content. In variousimplementations of the invention, the stepped profile may be optimizedby trying to maximize the volume or width of the middle portion of themetal gate electrode 102 while maintaining its electrical isolation frommisaligned trench contact 200. In some implementations, this may be doneby increasing the size or thickness of the metal gate electrode 102 b.In further implementations, this may be done by using more than twometal gate electrode layers to more finely tailor the stepped profile.

In accordance with another implementation of the invention, FIGS. 7A to7C illustrate the fabrication of a MOS transistor that combines the wideinsulator-cap layer 504 of FIG. 5F with the stepped profile metal gateelectrode 102 of FIGS. 6D to 6F. Starting with the structure shown inFIG. 5C, dual metal gate electrode layers are deposited as shown in FIG.7A. One layer is a conformal metal gate electrode layer 102 a and theother layer is a second metal layer 102 b that may or may not beconformal. The initial metal gate electrode layer 102 a is typically aworkfunction metal layer and the second metal gate electrode layer 102 bmay be a second workfunction metal layer or it may be a fill metallayer. In accordance with implementations of the invention, the metalused in the metal gate electrode 102 a has different etch propertiesthan the metal used in the metal gate electrode 102 b.

Moving to FIG. 7B, the dual metal gate electrode layers 102 a and 102 b,as well as the gate dielectric layer 104, are etched and recessed. Theetch process is selective to the metal gate electrode 102 b. This yieldsa stepped profile for the metal gate electrode 102, as shown in FIG. 7B.A middle portion of the overall metal gate electrode 102 is relativelythicker than the outer edge portions of the overall metal gate electrode102.

An insulating material is then deposited and planarized to forminsulator-cap layers 700 atop each metal gate electrode 102. This isshown in FIG. 7C. Also shown is a misaligned trench contact 200. Thestepped profile of the metal gate electrode 102 allows the thick portionof the insulator-cap layer 700 to electrically isolate the metal gateelectrode 102 from the trench contact 200. The stepped profile alsoallows a middle portion of the metal gate electrode 102 to remain thick,thereby reducing electrical resistance. In this implementation, theinsulating-cap layer 700 extends over the recessed spacers 108, therebyprotecting the spacers during the trench contact 200 etch process andallowing a material to be used in the spacers 108 that is optimized forreducing parasitic capacitance between the trench contact 200 and themetal gate electrode 102.

FIGS. 8A to 8F illustrate another implementation of the invention inwhich contact sidewall spacers are used to reduce CTG shorts and toimprove parasitic capacitance issues. FIG. 8A illustrates a contacttrench opening 800 that has been etched through ILD layers 110 a and 110b down to the diffusion region 106. As explained above, photolithographypatterning and etching processes are used to form the contact trenchopening 800.

Also shown in FIG. 8A is a silicide layer 802 that has been formed atthe bottom of the contact trench opening 800. To fabricate the silicidelayer 802, a conventional metal deposition process, such as a sputteringdeposition process or an ALD process, may be used to form a conformalmetal layer along at least the bottom of the contact trench opening 800.Often the metal will deposit on the sidewalls of the contact trenchopening 800 as well. The metal may include one or more of nickel,cobalt, tantalum, titanium, tungsten, platinum, palladium, aluminum,yttrium, erbium, ytterbium, or any other metal that is a good candidatefor a silicide. An annealing process may then be carried out to causethe metal to react with the diffusion region 106 and form a silicidelayer 802. Any unreacted metal may be selectively removed using knownprocesses. The silicide layer 802 reduces the electrical resistancebetween the later formed trench contact 200 and the diffusion region106.

FIG. 8B illustrates a pair of contact sidewall spacers 804 that areformed along the sidewalls of the contact trench opening 800, inaccordance with an implementation of the invention. The contact sidewallspacers 804 may be formed using deposition and etching processes similarto the fabrication of gate spacers 108. For instance, a conformal layerof an insulating material may be deposited within the contact trenchopening 800, resulting in the insulating material being deposited alongthe sidewalls and bottom surface of the contact trench opening 800. Theinsulating material may be silicon oxide, silicon nitride, siliconoxynitride (SiON), carbon-doped silicon oxynitride (SiOCN), any otheroxide, any other nitride, or any low-k dielectric material. Next, ananisotropic etching process is used to remove the insulating materialfrom the bottom of the contact trench opening 800, as well as from otherareas such as the surface of the ILD layer 110 b. This yields thecontact sidewall spacers 804 that are shown in FIG. 8B.

As will be appreciated by those of skill in the art, a separatepatterning process may be used to form vias down to the metal gateelectrodes 102 in order to form gate contacts. This separate patterningprocess will typically involve coating the wafer with a sacrificialphoto-definable resist layer, etching the gate contacts, and thenremoving the photoresist with a wet or dry cleaning process or somecombination thereof. This separate patterning process is generallycarried out after the contact trench opening 800 has been formed, whichmeans first the resist coating and then the wet or dry clean chemistryenters the contact trench opening 800 and can degrade the silicide layer802. Therefore, in accordance with an implementation of the invention,the conformal layer of insulating material used to form the spacers 804is deposited before the patterning process for the gate contacts. Theconformal layer remains in place to protect the silicide layer 802 untilafter the gate contacts have been patterned. Then the anisotropic etchdescribed above may be carried out to etch the conformal layer and formthe spacers 804.

It should be noted that the silicide layer 802 is formed prior tofabrication of the contact sidewall spacers 804, which is when thecontact trench opening 800 is at its largest width. By forming thesilicide layer 802 before forming the contact sidewall spacers 804, arelatively wider silicide layer 802 can be formed to provide betterelectrical resistance properties, such as lower intrinsic contactresistance. If the contact sidewall spacers 804 are formed first, thenless of the diffusion region 106 would be exposed for the silicidefabrication process, yielding a relatively shorter silicide layer.

A metal deposition process is then carried out to fill the contacttrench opening 800 and form the trench contact 200, as shown in FIG. 8C.As noted above, the metal deposition process can be any metal depositionprocess, such as electroplating, electroless plating, chemical vapordeposition, physical vapor deposition, sputtering, or atomic layerdeposition. The metal used may be any metal that provides suitablecontact properties, such as tungsten or copper. A metal liner is oftendeposited prior to the metal, such as a tantalum or tantulum nitrideliner. A CMP process is used to remove any excess metal and complete thefabrication of the trench contact 200.

The contact sidewall spacers 804 provide an additional layer ofprotection between the gate electrodes 102 and the trench contact 200.The final trench contact 200 has a relatively narrower width than trenchcontacts 200 formed using conventional processes, thereby reducing thelikelihood of CTG shorts. And the additional layer of insulation betweenthe gate electrodes 102 and the trench contact 200 reduces parasiticcapacitance.

FIGS. 8D to 8F illustrate the fabrication of contact sidewall spacers804 when the contact is misaligned. FIG. 8D illustrates a misalignedcontact trench opening 800 that has been etched through ILD layers 110 aand 110 b down to the diffusion region 106. The insulating-cap layer 300protects the metal gate electrode 102 from being exposed during thisetching process, in accordance with an implementation of the invention.Also shown in FIG. 8D is a silicide layer 802 that has been formed atthe bottom of the contact trench opening 800. Fabrication processes forthe silicide layer 802 were provided above.

FIG. 8E illustrates a pair of contact sidewall spacers 804 that areformed along the sidewalls of the contact trench opening 800, inaccordance with an implementation of the invention. The contact sidewallspacers 804 may be formed by depositing and etching a conformal layer ofan insulating material, as explained above.

A metal deposition process is then carried out to fill the contacttrench opening 800 and form the trench contact 200, as shown in FIG. 8F.Here again, the contact sidewall spacers 804 provide an additional layerof protection between the gate electrodes 102 and the trench contact200. The contact sidewall spacers 804 provide more separation betweenthe final trench contact 200 and the metal gate electrodes 102, therebyreducing the likelihood of CTG shorts. And the additional layer ofinsulation between the gate electrodes 102 and the trench contact 200reduces parasitic capacitance.

FIGS. 9A to 9D illustrate another process for forming an insulating-caplayer in accordance with an implementation of the invention. FIG. 9Aillustrates two MOS transistors having metal gate electrodes 102 andgate dielectric layer 104. The gate electrode layer 102 may include twoor more layers (not illustrated), such as a workfunction metal layer anda fill metal layer. Although the gate dielectric layer 104 showncorresponds to a replacement-metal gate process, the following processmay also be used with transistors formed using a gate-first approach.

A metal-cap 900 is formed atop the metal gate electrode 102, as shown inFIG. 9A. In accordance with implementations of the invention, themetal-cap 900 is formed using a selective deposition process. Someselective deposition processes include, but are not limited to,electroless plating and chemical vapor deposition. Metals that may beselectively deposited include, but are not limited to, cobalt, nickel,platinum, copper, polysilicon, tungsten, palladium, silver, gold, andother noble metals. As will be appreciated by those of skill in the art,the choice of whether an electroless process or a CVD process is usedwill depend on the composition of the metal gate electrode 102 and thespecific metal that is used in the metal-cap 900. In one example, if thetop portion of the metal gate electrode 102 consists of copper metal,then cobalt metal can be electrolessly deposited on the copper. Inanother example, tungsten or polysilicon can be deposited by CVD onalmost any metal that is used in the metal gate electrode 102. Inanother example, if the top portion of the metal gate electrode 102consists of a noble metal, then most metals may be deposited using anelectroless process on the noble metal. As will be appreciated by thoseof ordinary skill in the art, in general, electroless processes requirea noble metal for both the substrate metal and the metal to bedeposited. Therefore combinations of metals such as cobalt, nickel,copper, platinum, palladium, gold, and silver are possible.

Moving to FIG. 9B, an ILD layer 902 is blanket deposited over the ILD110 a and the metal-caps 900. A CMP process is then used to plagiarizeboth the ILD layer 902 and the metal-caps 900 and cause their topsurfaces to be substantially even. This is done to expose the topsurface of the metal-caps 900 after the ILD deposition.

Next, as shown in FIG. 9C, an etching process is used to remove themetal-caps 900 from within the ILD layer 902. In one implementation, awet etch chemistry may be applied to remove the metal-caps 900. Inaccordance with implementations of the invention, the etch chemistrythat is used must be selective to both the ILD layer 902 and the metalgate electrode 102. This enables the metal-caps 900 to be removed withminimal impact to the ILD layer 902 and the metal gate electrode 102.The removal of the metal-caps 900 yields voids 904 within the ILD layer902.

Moving to FIG. 9D, an insulating layer, such as a silicon nitride layer,may be deposited and planarized to fill in the voids 904, therebyforming self-aligned insulating-cap layers 906. This insulating layer isgenerally deposited as a blanket layer that fills the voids 904 andcovers the ILD layer 902. A planarization process is then used to removeany excess material that is outside of the voids 904. This confines theinsulating material to the voids 904, thereby forming insulating-caplayers 906. The insulator-cap layers 906 may be formed of materials thatinclude, but are not limited to, silicon nitride, silicon oxide, siliconcarbide, silicon nitride doped with carbon, silicon oxynitride, othernitride materials, other carbide materials, aluminum oxide, other oxidematerials, other metal oxides, and low-k dielectric materials. The onlyconstraint is that the material used in the insulator-cap layers 906 bedissimilar to the material used in the ILD layer 902.

FIGS. 10A to 10G illustrate a process for forming a self-aligned metalstud atop the trench contact 200 and a pair of insulating spacers thatfurther insulate the metal stud from the metal gate electrodes 102, inaccordance with an implementation of the invention. FIG. 10A illustratestwo MOS transistors having metal gate electrodes 102 and gate dielectriclayer 104. A trench contact 200 is formed between the two MOStransistors.

A metal-cap 900 is formed atop the trench contact 200, as shown in FIG.10A. In accordance with implementations of the invention, the metal-cap900 is formed using a selective deposition process. As noted above,selective deposition processes include, but are not limited to,electroless plating and chemical vapor deposition. The same metals andprocesses described above for use with the metal gate electrode 102 mayalso be used here with the trench contact 200. The selective depositionprocess used and the metal used in the metal-cap 900 will depend on themetal that is used in the trench contact 200.

In accordance with implementations of the invention, a selectivedeposition process is chosen that will deposit metal on only the trenchcontact 200 and not on the metal gate electrode 102. This can beaccomplished by using different types of metals in the trench contact200 and the metal gate electrode 102. For example, if aluminum is usedin the metal gate electrode 102 and a noble metal is used in the trenchcontact 200, then a selective deposition process can be used to depositthe metal-cap 900 on only the noble metal in the trench contact 200. Thesame combinations of noble metals described above will work here aswell. In some implementations of the invention, when an active metalsuch as aluminum, tungsten, molybdenum, titanium, tantalum, titaniumnitride, or polysilicon is used in the metal gate electrode 102, then anoble metal such as cobalt, nickel, copper, platinum, palladium, gold,and silver may be used in the trench contact 200.

Moving to FIG. 10B, an ILD layer 902 is blanket deposited over the ILD110 a and the metal-cap 900. A CMP process is then used to planarizeboth the ILD layer 902 and the metal-cap 900 and cause their topsurfaces to be substantially even. This is done to expose the topsurface of the metal-cap 900 after the ILD deposition.

Next, as shown in FIG. 10C, an etching process is used to remove justthe metal-cap 900 from within the ILD layer 902. The etch chemistry thatis used must be selective to both the ILD layer 902 and the trenchcontact 200. This enables the metal-cap 900 to be removed with minimalimpact to the ILD layer 902 and the trench contact 200. The removal ofthe metal-cap 900 yields a void 904 within the ILD layer 902.

Moving to FIG. 10D, an insulating layer 906 may be blanket depositedover the ILD layer 902 and within the void 904. The insulating layer 906may be formed of materials that include, but are not limited to, siliconnitride, silicon oxide, silicon carbide, silicon nitride doped withcarbon, silicon oxynitride, other nitride materials, other carbidematerials, aluminum oxide, other oxide materials, other metal oxides,and low-k dielectric materials, including materials that are the same orsimilar to the material used in the ILD layer 902.

Next, an etching process, such as an anisotropic etching process isapplied to etch down the insulating layer 906 and form spacers 1000.This is shown in FIG. 10E. The etching process also creates a trench1002 between the two spacers 1000.

Moving to FIG. 10F, a metal deposition process is used to deposit aself-aligned metal stud 1004 in the trench 1002 between the spacers 1000and atop the trench contact 200. In some implementations this metaldeposition process may be another selective deposition process, while inother implementations this metal deposition process need not be aselective process. Finally, as shown in FIG. 10G, an insulating layermay be deposited and planarized to form an ILD layer 1006. The top ofthe metal stud 1004 is also planarized to be even with the ILD layer1006. In accordance with implementations of the invention, the selfaligned metal stud 1004 is prevented from shorting to the gate by thespacers 1000.

Thus, implementations of the invention are described here that form etchstop structures that are self aligned to the gate, preventing thecontact etch from exposing the gate electrode to cause shorting betweenthe gate and contact. A contact to gate short is prevented even in thecase of the contact pattern overlaying the gate electrode.Implementations of the invention also address problems such as parasiticcapacitance between trench contacts and gate electrodes, dielectricbreakdown or direct shorts from contact to gate, and degradation ofcontact silicide during gate contact patterning.

Accordingly, the use of an insulator-cap layer enables self-alignedcontacts, which offer a robust manufacturable process. The inventionallows initial patterning of wider contacts which is more robust topatterning limitations. The wider contacts are also desirable for asilicide-through-contact process flow. Not only does this eliminate amajor yield limiter in contact-to-gate shorts, but it also alleviatesmajor constraints for contact patterning and allows for morevariability. From a lithography perspective, the use of an insulator-caplayer increases the registration window and allows for more criticaldimension variability. From an etch perspective, the use of aninsulator-cap layer makes the fabrication process for MOS transistorsmore tolerant to different profiles, different critical dimensions, andover-etching of the ILD during trench contact formation.

The above description of illustrated implementations of the invention,including what is described in the Abstract, is not intended to beexhaustive or to limit the invention to the precise forms disclosed.While specific implementations of, and examples for, the invention aredescribed herein for illustrative purposes, various equivalentmodifications are possible within the scope of the invention, as thoseskilled in the relevant art will recognize.

These modifications may be made to the invention in light of the abovedetailed description. The terms used in the following claims should notbe construed to limit the invention to the specific implementationsdisclosed in the specification and the claims. Rather, the scope of theinvention is to be determined entirely by the following claims, whichare to be construed in accordance with established doctrines of claiminterpretation.

1. A transistor comprising: a substrate; a pair of spacers on thesubstrate; a gate dielectric layer on the substrate and between the pairof spacers; a gate electrode layer on the gate dielectric layer andbetween the pair of spacers; an insulating cap layer on the gateelectrode layer and between the pair of spacers; and a pair of diffusionregions adjacent to the pair of spacers.
 2. The transistor of claim 1,wherein a height of the combination of the gate dielectric layer, thegate electrode layer, and the insulating cap layer does not exceed aheight of the pair of spacers.
 3. The transistor of claim 1, wherein thegate dielectric layer is formed along the surface of the substrate andalong sidewalls of the pair of spacers.
 4. The transistor of claim 1,wherein the insulating cap layer comprises silicon nitride, siliconoxide, silicon carbide, silicon nitride doped with carbon, siliconoxynitride, or aluminum oxide.
 5. The transistor of claim 1, wherein theinsulating cap layer comprises a nitride material, a carbide material,an oxide material, a metal oxide material, or a low-k dielectricmaterial.
 6. The transistor of claim 1, wherein the insulating cap layercomprises boron nitride or boron carbide.
 7. The transistor of claim 1,wherein the insulating cap layer comprises a low-k dielectric materialdoped with one or more of carbon, nitrogen, and hydrogen.
 8. A method offorming a transistor comprising: forming a gate dielectric layer on asubstrate; forming a gate electrode layer on the gate dielectric layer;forming a pair of spacers on opposing sides of the gate dielectric layerand the gate electrode layer; forming a pair of diffusion regionsadjacent to the pair of spacers; recessing the gate electrode layer; andforming an insulating cap layer on the recessed gate electrode layerwithin the pair of spacers.
 9. The method of claim 8, wherein theforming of the insulating cap layer comprises: depositing a blanketlayer of an insulating material over the recessed gate electrode layer;and planarizing the insulating material layer to form the insulating caplayer.
 10. The method of claim 8, further comprising: removing the gateelectrode layer and the gate dielectric layer after the diffusionregions have been formed, thereby forming a trench between the pair ofspacers; depositing a conformal low-k gate dielectric layer along thesidewalls and bottom surface of the trench between the spacers; anddepositing a metal gate electrode layer on the low-k gate dielectriclayer before recessing the metal gate electrode.
 11. The method of claim8, wherein the insulating cap layer comprises silicon nitride, siliconoxide, silicon carbide, silicon nitride doped with carbon, siliconoxynitride, or aluminum oxide.
 12. The method of claim 8, wherein theinsulating cap layer comprises a nitride material, a carbide material,an oxide material, a metal oxide material, or a low-k dielectricmaterial.
 13. The method of claim 8, wherein the insulating cap layercomprises boron nitride or boron carbide.
 14. The method of claim 8,wherein the insulating cap layer comprises a low-k dielectric materialdoped with one or more of carbon, nitrogen, and hydrogen.
 15. A methodof forming a transistor comprising: forming a gate stack on a substrate,the gate stack comprising a gate dielectric layer, a gate electrodelayer on the gate dielectric layer, and an insulating cap layer on thegate electrode layer; forming a pair of spacers on opposing sides of thegate stack; and forming a pair of diffusion regions adjacent to the pairof spacers;
 16. The method of claim 15, wherein the insulating cap layercomprises silicon nitride, silicon oxide, silicon carbide, siliconnitride doped with carbon, silicon oxynitride, or aluminum oxide. 17.The method of claim 15, wherein the insulating cap layer comprises anitride material, a carbide material, an oxide material, a metal oxidematerial, or a low-k dielectric material.
 18. The method of claim 15,wherein the insulating cap layer comprises boron nitride or boroncarbide.
 19. The method of claim 15, wherein the insulating cap layercomprises a low-k dielectric material doped with one or more of carbon,nitrogen, and hydrogen.
 20. A transistor comprising: a substrate; a pairof spacers on the substrate; a gate dielectric layer on the substrateand between the pair of spacers; a gate electrode layer on the gatedielectric layer and between the pair of spacers; an insulating caplayer sited atop the gate electrode layer that extends laterally overthe top surfaces of the pair of spacers; and a pair of diffusion regionsadjacent to the pair of spacers.
 21. The transistor of claim 20, whereinthe gate dielectric layer is formed along the surface of the substrateand along sidewalls of the pair of spacers.
 22. The transistor of claim20, wherein the insulating cap layer comprises silicon nitride, siliconoxide, silicon carbide, silicon nitride doped with carbon, siliconoxynitride, or aluminum oxide.
 23. The transistor of claim 20, whereinthe insulating cap layer comprises a nitride material, a carbidematerial, an oxide material, a metal oxide material, or a low-kdielectric material.
 24. The transistor of claim 20, wherein theinsulating cap layer comprises boron nitride or boron carbide.
 25. Thetransistor of claim 20, wherein the insulating cap layer comprises alow-k dielectric material doped with one or more of carbon, nitrogen,and hydrogen.
 26. The transistor of claim 20, wherein the pair ofspacers comprises silicon oxynitride, carbon-doped silicon oxynitride,or a low-k dielectric material.
 27. A method of forming a transistorcomprising: forming a gate dielectric layer on a substrate; forming agate electrode layer on the gate dielectric layer; forming a pair ofspacers on opposing sides of the gate dielectric layer and the gateelectrode layer; forming a pair of diffusion regions adjacent to thepair of spacers; recessing the gate electrode layer; recessing the pairof spacers; and forming an insulating cap layer on the recessed gateelectrode layer that extends laterally over the top surfaces of therecessed pair of spacers.
 28. The method of claim 27, furthercomprising: removing the recessed gate electrode layer and the gatedielectric layer after the spacers have been recessed, thereby forming atrench between the recessed pair of spacers; depositing a conformallow-k gate dielectric layer along the sidewalls and bottom surface ofthe trench between the recessed pair of spacers; depositing a metal gateelectrode layer on the low-k gate dielectric layer; and recessing themetal gate electrode so that a height of the recessed metal gateelectrode is similar to a height of the recessed pair of spacers beforethe insulating cap layer is formed.
 29. The method of claim 27, whereinthe forming of the insulating cap layer comprises: depositing a blanketlayer of an insulating material over the recessed gate electrode layerand recessed pair of spacers; and planarizing the insulating materiallayer to form the insulating cap layer.
 30. The method of claim 27,wherein a height of the recessed pair of spacers is substantially thesame as a height of the recessed gate electrode.
 31. The method of claim27, wherein the pair of spacers comprises silicon oxynitride (SiON),carbon-doped silicon oxynitride (SiOCN), or a low-k dielectric material.32. A transistor comprising: a substrate; a pair of spacers on thesubstrate; a gate dielectric layer on the substrate and between the pairof spacers; a gate electrode layer having a stepped profile on the gatedielectric layer, wherein a middle portion of the gate electrode layerhas a relatively larger height than side portions of the gate electrodelayer; an insulating cap layer on the gate electrode layer and betweenthe pair of spacers; and a pair of diffusion regions adjacent to thepair of spacers.
 33. The transistor of claim 32, wherein the insulatingcap layer comprises silicon nitride, silicon oxide, silicon carbide,silicon nitride doped with carbon, silicon oxynitride, or aluminumoxide.
 34. The transistor of claim 32, wherein the insulating cap layercomprises a nitride material, a carbide material, an oxide material, ametal oxide material, or a low-k dielectric material.
 35. The transistorof claim 32, wherein the insulating cap layer comprises boron nitride orboron carbide.
 36. The transistor of claim 32, wherein the insulatingcap layer comprises a low-k dielectric material doped with one or moreof carbon, nitrogen, and hydrogen.
 37. The transistor of claim 32,wherein the gate electrode comprises two metal layers, wherein a firstmetal layer comprises the middle portion of the gate electrode havingthe relatively larger height and a second metal layer comprises the sideportions of the gate electrode.
 38. A method of forming a transistorcomprising: forming a gate dielectric layer on a substrate; forming agate electrode layer on the gate dielectric layer; forming a pair ofspacers on opposing sides of the gate dielectric layer and the gateelectrode layer; forming a pair of diffusion regions adjacent to thepair of spacers; removing the gate electrode layer and the gatedielectric layer, thereby forming a trench between the pair of spacers;depositing a conformal first metal layer within the trench; depositing asecond metal layer on the first metal layer; recessing the first andsecond metal layers, wherein the first metal layer is recessed to alarger extent than the second metal layer; and forming an insulating caplayer on the recessed first and second metal layers.
 39. The method ofclaim 38, wherein the recessing of the first and second metal layerscomprises: applying an etch chemistry to the first and second metallayers that is selective to the second metal layer.
 40. The method ofclaim 38, wherein the recessing of the first and second metal layerscomprises: applying a first etch chemistry to recess the first metallayer; and applying a second etch chemistry to recess the second metallayer, wherein the first and second etch chemistries recess the firstmetal layer to a larger extent than the second metal layer.
 41. Themethod of claim 40, wherein the second etch chemistry also recesses thefirst metal layer.
 42. The method of claim 38, wherein the forming ofthe insulating cap layer comprises: depositing a blanket layer of aninsulating material over the recessed first and second metal layers; andplanarizing the insulating material layer to form the insulating caplayer.
 43. The method of claim 38, wherein the insulating cap layercomprises silicon nitride, silicon oxide, silicon carbide, siliconnitride doped with carbon, silicon oxynitride, or aluminum oxide. 44.The method of claim 38, wherein the insulating cap layer comprises anitride material, a carbide material, an oxide material, a metal oxidematerial, or a low-k dielectric material.
 45. The method of claim 38,wherein the insulating cap layer comprises boron nitride or boroncarbide.
 46. The method of claim 38, wherein the insulating cap layercomprises a low-k dielectric material doped with one or more of carbon,nitrogen, and hydrogen.
 47. A transistor comprising: a substrate; a pairof spacers on the substrate; a gate dielectric layer on the substrateand between the pair of spacers; a gate electrode layer having a steppedprofile on the gate dielectric layer, wherein a middle portion of thegate electrode layer has a relatively larger height than side portionsof the gate electrode layer; an insulating cap layer sited atop the gateelectrode layer that extends laterally over the top surfaces of the pairof spacers; and a pair of diffusion regions adjacent to the pair ofspacers.
 48. The transistor of claim 47, wherein the insulating caplayer comprises silicon nitride, silicon oxide, silicon carbide, siliconnitride doped with carbon, silicon oxynitride, or aluminum oxide. 49.The transistor of claim 47, wherein the insulating cap layer comprises anitride material, a carbide material, an oxide material, a metal oxidematerial, or a low-k dielectric material.
 50. The transistor of claim47, wherein the insulating cap layer comprises boron nitride or boroncarbide.
 51. The transistor of claim 47, wherein the insulating caplayer comprises a low-k dielectric material doped with one or more ofcarbon, nitrogen, and hydrogen.
 52. The transistor of claim 47, whereinthe gate electrode comprises two metal layers, wherein a first metallayer comprises the middle portion of the gate electrode having therelatively larger height and a second metal layer comprises the sideportions of the gate electrode.